These topics also covered in your book p.150 - 154.

Using your web browser, go to:

http://vcourseware.calstatela.edu/VirtualEarthquake/

(Copyright © 1996, 1999 Geology Labs On-Line. Gary Novak, Department of Geological Sciences. California State University, Los Angeles, CA. 90032-8201) but they want everyone to use it!

And follow along with their instructions. Text in this homework in quotation marks is text from their website.

Welcome to Virtual Earthquake

"Virtual Earthquake is an interactive Web-based activity designed to introduce you to the concepts of how an earthquake EPICENTER is located and how the RICHTER MAGNITUDE of an earthquake is determined. The Virtual Earthquake program is running on a Web Server at California State University at Los Angeles. You can interact with Virtual Earthquake using either a Netscape or Internet Explorer Web Browser running on Macs or PCs." Scroll down this first page and click on the Execute button to start the Virtual Earthquake application.

1. Why do earthquakes occur?

2. Where do they occur?

3. What is a focus?

4. What are earthquake (seismic) waves?

5. How fast do P and S waves travel?

6. What's a Seismogram? (also see fig. 6.7 in book)

Study the sample seismogram on the web page and be sure you can identify all the parts - you will need to identify these and make measurements from them.

7. How is an earthquake's epicenter located?

OK. Let's have an earthquake!

Choose any one of the following regions to generate a set of seismograms for an earthquake: 1.San Francisco area 2. Southern California 3. Japan region 4. Mexico (not recommended) Click on "Submit Choice."

The next page instructs you on how to read and use the seismogram. Read it carefully. Click on "Review Seismograms".

Determining The Earthquake Epicenter

Use the three seismograms presented on the web page to estimate the S-P time interval for each of the recording stations. Type in your measurement for the S-P interval in the box below each seismogram. Whole numbers (no decimals) are best.

When you have input numbers into all three boxes, click on "Convert S-P Interval."

Determining the Earthquake Distance

"You can now determine the distance from each seismic recording station to the earthquake's epicenter using the known times of travel of the S and P waves."

8. It takes an S wave approximately ___ seconds to travel ____ kilometers. Study the graph to make sure you understand how they came up with these numbers.

9. For practice, how long does it take the P wave to travel this same distance?

For the rest of this exercise you won't be needing the individual S and P curves, only the S-P curve. To determine the epicentral distance, we need greater resolution on graph. We will use the graph below, which shows an expanded part of the S-P curve.

"Use the S-P graph to the left and the estimates you made for the S-P time intervals for the three seismograms (not shown on my computer) to complete the table given." YOU MUST DO SOME GRAPH READING HERE!

Fill in the table, then click on "Find Epicenter".

Triangulation of the Epicenter

10. What you have done is use the records from three stations to triangulate the position of the earthquake. How well did you do? Do your three circles intersect at one point? (I got an 'excellent' and a 'very close' on two tries - donŐt be too concerned if they suggest that you need to go back and re-measure.)

The Actual Epicenter

11. Why may this method not produce an exact point? (besides measurement and graph-reading errors)

The epicenter of this earthquake has been located. Now let's determine its Richter magnitude.

Click on "Compute Richter Magnitude".

Magnitude Explained

12. The magnitude of an earthquake is an estimate of:

13. The Richter magnitude determination is based on:

14. Two measurements are needed:

Look at their illustration of measuring amplitude and make sure you understand it. Then click on "go to next page".

The Richter Nomogram

"Although the relationship between Richter magnitude and the measured amplitude and S-P interval is complex, a graphical device (a nomogram) can be used to simplify the process and to estimate magnitude from distance and amplitude."

15. "Note that a 100 km-away earthquake of magnitude ___ would produce ___ mm of amplitude and a magnitude ___ would produce ____ mm of amplitude: _______________ are all powers of 10 and this is why the Richter Scale is said to be "_______________." A change of _________ in magnitude (say from 4 to 5) increases the maximum ______________ by a factor of____."

Click on "Go to the Next Page."

"Below are the three seismograms of the earthquake you are studying. Measure the maximum amplitude of the S-wave for each seismogram and record your estimate in the box below the seismogram. Note that although only one amplitude measurement is necessary, you should measure the amplitude for each of the three stations. This will enable you to determine the magnitude value as an average of three values, thus increasing the likelihood that you are accurate in your estimate."

Input your 3 numbers and click on "Submit to Nomogram."

"Below is Richter's nomogram with three lines representing the data you provided. Use this diagram and estimate the Richter magnitude of this earthquake. (Your three lines might not cross at a point on the magnitude scale.) Enter your estimate in the box below the nomogram."

16. My estimate for the Richter magnitude of this earthquake = ______

Click on "Confirm Magnitude".

17. How did you do? What historic earthquake was yours modeled on? Its estimated magnitude was______. How did your estimate compare to this value?

If you would like to receive a Certificate of Completion as a Virtual Seismologist, complete the form below and click the "Get Certificate" button. This is not necessary however!!!

A real (and nearby) example

A magnitude 4.5 earthquake happened near Bardwell Kentucky (extreme western KY) on June 6, 2003. The Lamont-Doherty Cooperative Seismographic Network (LCSN) near New York City recorded it. Take a look at their information at http://www.ldeo.columbia.edu/res/pi/LCSN/Eq/20030606/20030606_1229.html.

Scroll down to see the seismogram recorded in New York. A blip, but not as clean as the ones we worked with above. Can you see the arrival of the P and S waves? The P wave amplitude is very small.

18. Scroll down more to see the "Seismic Record Section". Here a number of seismograms are lined up on a graph of distance vs. time. Can you see the different arrival times of the P and S waves in this graph? At great distances from the epicenter, does the time between the arrival of the P wave and the S wave increase, decrease, or stay the same?

On a sadder note

The Lamont-Doherty Cooperative Seismographic Network (LCSN) also recorded the seismic signal of the impact and collapse of both World Trade Center towers on Sept. 11, 2001.

http://www.ldeo.columbia.edu/res/pi/LCSN/Eq/20010911_wtc.html

Last updated: Feb. 28, 2005

koziol@notes.udayton.edu SCI 210 syllabus Dr. Koziol's homepage UD Dept. of Geology Home

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